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Is there any way to check whether an integer is a member of a list of integer numbers and return a boolean?

I found this neat solution, here that does a great job. It checks whether a string exists in a list of strings:

documentclassarticle
usepackagexstring
newcommandIfStringInList[2]IfSubStr,#2,,#1,
begindocument
 IfStringInListPaulGeorge,John,Paul,RingoTrueFalse
enddocument

However, it does return a string not a boolean "true" or "false" and caused me some difficulty which is posted here.

Let's call the macro I would like to have ISMEMBER. My goal is to use ISMEMBER and examine whether an integer is in the list and depending on whether it is or not, perform some tasks, as an example:

fpeval ISMEMBER11,2,3,4,5 ? MACRO_FOR_MEMBERS : MACRO_FOR_NON_MEMBERS 

Is such functionality possible?

You can use do a comma-separated list parser to loop through the list of items and check if the given item is present in the list. Since the comma-separated list can be read by using a delimited macro, this can be done expandably, allowing you to plug the macro into fpeval to get the syntax you want.

Using expl3 (you are loading it with xfp anyway) something like this would solve the problem:

ExplSyntaxOn
prg_new_conditional:Npnn afp_int_ismember:nn #1#2 p, T, F, TF 
 __afp_ismember_loop:nw #1 #2 , q_recursion_tail , q_recursion_stop 
cs_new:Npn __afp_ismember_loop:nw #1#2,
 
 quark_if_recursion_tail_stop_do:nn #2
 prg_return_false: 
 int_compare:nNnTF #1 = #2
 use_i_delimit_by_q_recursion_stop:nw prg_return_true: 
 __afp_ismember_loop:nw #1 
 
ExplSyntaxOff

The code above defines a conditional afp_int_ismember:nn(TF), whose first argument is the item to check, and the second is the comma-separated list. The macro starts by expanding __afp_ismember_loop:nw: this macro takes the item to be tested (#1) and the first item in the list, delimited by a , (#2). The macro tests the equality of #1 and #2 using int_compare:nNnTF and issues prg_return_true: if they are equal or calls __afp_ismember_loop:nw for the next item. If the end of the list is found (i.e., q_recursion_tail is grabbed), then the function issues prg_return_false: because no match was found for #1.

The code above can be tweaked to replace int_compare:nNnTF by a generic equality comparison function. If you do that you can define wrappers around __afp_ismember_loop:nw to create ismember functions for different data types. The code below does that and defines two functions: afp_int_ismember:nn(TF) (using int_compare:nNnTF) and afp_str_ismember:nn(TF) (using str_if_eq:eeTF). Doing that you can even test if I'm a member of the Beatles!

To use the function in fpeval you just need the predicate form (afp_int_ismember_p:nn) of the conditional function:

fpeval afp_int_ismember_p:nn 1 1,2,3,4,5 ? 123 : 321 

And the string version would also work here:

fpeval afp_str_ismember_p:nn Phelype George,John,Paul,Ringo ? 123 : 321 

Full code:

documentclassarticle
usepackageexpl3
usepackagexparse
usepackagexfp
ExplSyntaxOn
% Core code for the membership test
cs_new:Npn __afp_ismember_loop:Nnw #1#2#3,
 
 quark_if_recursion_tail_stop_do:nn #3
 prg_return_false: 
 #1 #2 #3
 use_i_delimit_by_q_recursion_stop:nw prg_return_true: 
 __afp_ismember_loop:Nnw #1 #2 
 
% Wrapper for testing integers
prg_new_conditional:Npnn afp_int_ismember:nn #1#2 p, T, F, TF 
 
 __afp_ismember_loop:Nnw __afp_int_isequal:nnTF #1 #2 ,
 q_recursion_tail , q_recursion_stop
 
prg_new_conditional:Npnn __afp_int_isequal:nn #1#2 p, T, F, TF 
 
 int_compare:nNnTF #1 = #2
 prg_return_true: 
 prg_return_false: 
 
% Wrappers for testing strings
 % With expansion
prg_new_conditional:Npnn afp_str_ismember:ee #1#2 p, T, F, TF 
 
 __afp_ismember_loop:Nnw str_if_eq:eeTF #1 #2 ,
 q_recursion_tail , q_recursion_stop
 
 % Without expansion
prg_new_conditional:Npnn afp_str_ismember:nn #1#2 p, T, F, TF 
 
 __afp_ismember_loop:Nnw str_if_eq:nnTF #1 #2 ,
 q_recursion_tail , q_recursion_stop
 
% Sample commands
NewExpandableDocumentCommand IntIsmember m m 
 
 afp_int_ismember:nnTF #1 #2
 #1~is~member~of~`#2' 
 #1~is~emphnot~member~of~`#2' 
 
NewExpandableDocumentCommand StrIsmember m m 
 
 afp_str_ismember:nnTF #1 #2
 #1~is~member~of~`#2' 
 #1~is~emphnot~member~of~`#2' 
 
ExplSyntaxOff
begindocument
IntIsmember11,2,3

IntIsmember41,2,3

StrIsmemberPaulGeorge,John,Paul,Ringo

textbfStrIsmemberPhelypeGeorge,John,Paul,Ringo

ExplSyntaxOn
fpeval afp_int_ismember_p:nn 1 1,2,3,4,5 ? 123 : 321 par
fpeval afp_str_ismember_p:nn Phelype George,John,Paul,Ringo ? 123 : 321 
ExplSyntaxOff
enddocument

Something like this? The macro name is inspired by the Mathematica command MemberQ, and the code comes from here. This solution does not require any packages.

documentclassarticle
newififmember
makeatletter% for @for see e.g. https://tex.stackexchange.com/a/100684/121799
%from https://tex.stackexchange.com/a/498576/121799
newcommandMemberQ[2]globalmemberfalse%
@fornext:=#1doifnumnext=#2globalmembertruefi
makeatother
begindocument

MemberQ1,2,3,42
ifmember 2 is in list fi

MemberQ1,2,3,45
ifmember 5 is in listelse%
5 is not in the listfi
enddocument